FIG. 1 illustrates the basic Long Term Evolution (LTE) downlink physical resource. LTE uses Orthogonal Frequency Division Multiplexing (OFDM) in the downlink and Discrete Fourier Transform (DFT)-spread OFDM in the uplink. The basic LTE downlink physical resource can thus be seen as a time-frequency grid, where each resource element (or time/frequency resource element, TFRE) corresponds to one OFDM subcarrier during one OFDM symbol interval.
FIG. 2 illustrates the LTE time-domain structure. In the time domain, LTE downlink transmissions are organized into radio frames of 10 ms. Each radio frame consists of ten equally-sized subframes of length Tsubframe=1 ms.
Furthermore, the resource allocation in LTE is typically described in terms of resource blocks (RBs), where a resource block corresponds to one slot (0.5 ms) in the time domain and 12 contiguous subcarriers in the frequency domain. Resource blocks are numbered in the frequency domain, starting with 0 from one end of the system bandwidth.
FIG. 3 illustrates an example downlink subframe. Downlink transmissions are dynamically scheduled. In other words, in each subframe the base station transmits control information about to which terminals data is transmitted and upon which resource blocks the data is transmitted in the current downlink subframe. This control signaling is typically transmitted in the first 1, 2, 3 or 4 OFDM symbols in each subframe. For example, FIG. 3 illustrates a downlink system with 3 OFDM symbols as control.
LTE uses hybrid-ARQ, where, after receiving downlink data in a subframe, the terminal attempts to decode it and reports to the base station whether the decoding was successful. If the decoding is successful, the terminal reports an acknowledgement (ACK) to the base station. Conversely, if the decoding is not successful, the terminal reports an negative acknowledgement (NAK) to the base station. In case of an unsuccessful decoding attempt, the base station can retransmit the erroneous data.
Uplink control signaling from the terminal to the base station includes hybrid-ARQ acknowledgements for received downlink data. Uplink control signaling may also include terminal reports related to the downlink channel conditions, used as assistance for the downlink scheduling. Additionally, uplink control signaling may include scheduling requests, indicating that a mobile terminal needs uplink resources for uplink data transmissions. If the mobile terminal has not been assigned an uplink resource for data transmission, the L1/L2 control information (Layer-1/Layer-2 control information, e.g., channel state information (CSI) reports, hybrid-ARQ acknowledgments, and scheduling requests) is transmitted in uplink resources (resource blocks) specifically assigned for uplink L1/L2 control on the Physical Uplink Control Channel (PUCCH).
FIG. 4 illustrates uplink L1/L2 control signaling transmission on PUCCH. The uplink resources assigned for uplink L1/L2 control on the PUCCH are located at the edges of the total available cell bandwidth. Each such resource consists of twelve subcarriers (one resource block) within each of the two slots of an uplink subframe. In order to provide frequency diversity, these frequency resources are frequency hopping on the slot boundary (i.e., one “resource” consists of 12 subcarriers at the upper part of the spectrum within the first slot of a subframe and an equally sized resource at the lower part of the spectrum during the second slot of the subframe or vice versa). If more resources are needed for the uplink L1/L2 control signaling, for example in the case of very large overall transmission bandwidth supporting a large number of users, additional resource blocks can be assigned next to the previously assigned resource blocks.
As described above, uplink L1/L2 control signaling includes hybrid-ARQ acknowledgements, channel state information reports and scheduling requests. Different combinations of these types of messages are possible as described below, but to explain the structure for these cases it is beneficial to discuss separate transmission of each of the types first, starting with the hybrid-ARQ and the scheduling request. There are three formats defined for PUCCH, each capable of carrying a different number of bits. A brief description of PUCCH format 2 is provided below.
In PUCCH format 2, channel state information reports are used to provide the eNodeB with an estimate of the channel properties at the terminal in order to aid channel-dependent scheduling. A channel state information report consists of multiple bits per subframe. PUCCH format 1, which is capable of at most two bits of information per subframe, may not be suitable for this purpose. Transmission of channel state information reports on the PUCCH is instead handled by PUCCH format 2, which is capable of multiple information bits per subframe. There are three variants in the LTE specifications: formats 2; 2a; and 2b. Formats 2a and 2b are used for simultaneous transmission of hybrid-ARQ acknowledgements (described in more detail below). For simplicity, they may all referred to as format 2 herein. The PUCCH format 2 resources are semi-statically configured.
Multi-antenna techniques can significantly increase the data rates and reliability of a wireless communication system. The performance is in particular improved if both the transmitter and the receiver are equipped with multiple antennas, which results in a multiple-input multiple-output (MIMO) communication channel. Such systems and/or related techniques are commonly referred to as MIMO.
The LTE standard is currently evolving with enhanced MIMO support. A core component in LTE is the support of MIMO antenna deployments and MIMO related techniques. LTE-Advanced supports an 8-layer spatial multiplexing mode for 8 transmit (Tx) antennas with channel dependent precoding. The spatial multiplexing mode is aimed for high data rates in favorable channel conditions.
FIG. 5 illustrates an example of spatial multiplexing operation. More particularly, FIG. 5 illustrates an example transmission structure of precoded spatial multiplexing mode in LTE. As depicted, the information carrying symbol vector s is multiplied by an NT×r precoder matrix W, which serves to distribute the transmit energy in a subspace of the NT (corresponding to NT antenna ports) dimensional vector space. The precoder matrix is typically selected from a codebook of possible precoder matrices, and typically indicated by means of a precoder matrix indicator (PMI), which specifies a unique precoder matrix in the codebook for a given number of symbol streams. The r symbols in s each correspond to a layer and r is referred to as the transmission rank. In this way, spatial multiplexing is achieved since multiple symbols can be transmitted simultaneously over the same time/frequency resource element (TFRE). The number of symbols r is typically adapted to suit the current channel properties.
LTE uses OFDM in the downlink (and DFT precoded OFDM in the uplink) and hence the received NR×1 vector yn over NR receiving antenna ports for a certain TFRE on subcarrier n (or alternatively data TFRE number n) is thus modeled by:yn=HnWsn+en where Hn is the channel matrix between eNodeB and a UE, W is the precoding matrix, sn is the transmitted symbol vector, and en is a noise/interference vector obtained as realizations of a random process. The precoder, W, can be a wideband precoder, which is constant over frequency, or frequency selective (i.e., different precoders on different subbands).
The precoder matrix is often chosen to match the characteristics of the NR×NT MIMO channel matrix Hn, resulting in so-called channel dependent precoding. This is also commonly referred to as closed-loop precoding, and essentially strives for focusing the transmit energy into a subspace which is strong in the sense of conveying much of the transmitted energy to the UE. In addition, the precoder matrix may also be selected to strive for orthogonalizing the channel, meaning that after proper linear equalization at the UE, the inter-layer interference is reduced.
The transmission rank, and thus the number of spatially multiplexed layers, is reflected in the number of columns of the precoder. For efficient performance, it is important that a transmission rank that matches the channel properties is selected.
In LTE Release 10, a new reference symbol sequence was introduced for estimating channel state information, the Channel State Information Reference Signal (CSI-RS). The CSI-RS provides several advantages over basing the CSI feedback on the cell specific reference signals (CRS), which were used for that purpose in previous releases. First, the CSI-RS is not used for demodulation of the data signal, and thus does not require the same density (i.e., the overhead of the CSI-RS is substantially less). Second, CSI-RS provides a much more flexible means to configure CSI feedback measurements (e.g., which CSI-RS resource to measure on can be configured in a UE specific manner).
By measuring on a CSI-RS, a UE can estimate the effective channel the CSI-RS is traversing (including the radio propagation channel and antenna gains). This implies that if a known CSI-RS signal X is transmitted, a UE can estimate the coupling between the transmitted signal and the received signal (i.e., the effective channel). Hence if no virtualization is performed in the transmission, the received signal y can be expressed as:y=Hx+e and the UE can estimate the effective channel H.
Up to eight CSI-RS ports can be configured for a Release 11 UE. That is, the UE can thus estimate the channel from up to eight transmit antennas.
FIGS. 6A-6C illustrate resource element grids. More particularly, FIGS. 6A-6C illustrate resource element grids over an RB pair showing potential positions for Release 9/10 UE specific RS, CSI-RS (marked with a number corresponding to the CSI-RS antenna port), and CRS. The CSI-RS utilizes an orthogonal cover code (OCC) of length two to overlay two antenna ports on two consecutive REs. As shown in FIGS. 6A-6C, many different CSI-RS patterns are available. For the case of 2 CSI-RS antenna ports, we see that there are 20 different patterns within a subframe. The corresponding number of patterns is 10 and 5 for 4 and 8 CSI-RS antenna ports, respectively. For TDD, some additional CSI-RS patterns are available.
The CSI reference signal configurations are shown in TABLE 6.10.5.2-1 below, taken from TS 36.211 v.12.5.0. For example, the CSI RS configuration 5 for 4 antennas ports use (k′,l′)=(9,5) in slot 1 (the second slot of the subframe). Using the formulas below, it can be determined that port 15,16, use OCC over the resource elements (k,l)=(9,5), (9,6) and ports 17,18 use OCC over resource elements (3,5), (3,6), respectively (assuming PRB index m=0), where k is the subcarrier index and l is the OFDM symbol index within each slot.
The orthogonal cover code (OCC) is introduced below by the factor w1′.
          ⁢      k    =                  k        ′            +              12        ⁢        m            +              {                                                                                                  -                    0                                                                                                                                      for                        ⁢                                                                                                  ⁢                        p                                            ∈                                              {                                                  15                          ,                          16                                                }                                                              ,                                          normal                      ⁢                                                                                          ⁢                      cyclic                      ⁢                                                                                          ⁢                      prefix                                                                                                                                        -                    6                                                                                                                                      for                        ⁢                                                                                                  ⁢                        p                                            ∈                                              {                                                  17                          ,                          18                                                }                                                              ,                                          normal                      ⁢                                                                                          ⁢                      cyclic                      ⁢                                                                                          ⁢                      prefix                                                                                                                                        -                    1                                                                                                                                      for                        ⁢                                                                                                  ⁢                        p                                            ∈                                              {                                                  19                          ,                          20                                                }                                                              ,                                          normal                      ⁢                                                                                          ⁢                      cyclic                      ⁢                                                                                          ⁢                      prefix                                                                                                                                        -                    7                                                                                                                                      for                        ⁢                                                                                                  ⁢                        p                                            ∈                                              {                                                  21                          ,                          22                                                }                                                              ,                                          normal                      ⁢                                                                                          ⁢                      cyclic                      ⁢                                                                                          ⁢                      prefix                                                                                                                                        -                    0                                                                                                                                      for                        ⁢                                                                                                  ⁢                        p                                            ∈                                              {                                                  15                          ,                          16                                                }                                                              ,                                          extended                      ⁢                                                                                          ⁢                      cyclic                      ⁢                                                                                          ⁢                      prefix                                                                                                                                        -                    3                                                                                                                                      for                        ⁢                                                                                                  ⁢                        p                                            ∈                                              {                                                  17                          ,                          18                                                }                                                              ,                                          extended                      ⁢                                                                                          ⁢                      cyclic                      ⁢                                                                                          ⁢                      prefix                                                                                                                                        -                    6                                                                                                                                      for                        ⁢                                                                                                  ⁢                        p                                            ∈                                              {                                                  19                          ,                          20                                                }                                                              ,                                          extended                      ⁢                                                                                          ⁢                      cyclic                      ⁢                                                                                          ⁢                      prefix                                                                                                                                        -                    9                                                                                                                                      for                        ⁢                                                                                                  ⁢                        p                                            ∈                                              {                                                  21                          ,                          22                                                }                                                              ,                                          extended                      ⁢                                                                                          ⁢                      cyclic                      ⁢                                                                                          ⁢                      prefix                                                                                            ⁢                                                  ⁢            l                    =                                    l              ′                        +                          {                                                                                                                                            l                          ″                                                                                                                                                  CSI                            ⁢                                                                                                                  ⁢                            reference                            ⁢                                                                                                                  ⁢                            signal                            ⁢                                                                                                                  ⁢                            configurations                            ⁢                                                                                                                  ⁢                            0                            ⁢                                                          -                                                        ⁢                            19                                                    ,                                                      normal                            ⁢                                                                                                                  ⁢                            cyclic                            ⁢                                                                                                                  ⁢                            prefix                                                                                                                                                                                        2                          ⁢                                                      l                            ″                                                                                                                                                                            CSI                            ⁢                                                                                                                  ⁢                            reference                            ⁢                                                                                                                  ⁢                            signal                            ⁢                                                                                                                  ⁢                            configurations                            ⁢                                                                                                                  ⁢                            20                            ⁢                                                          -                                                        ⁢                            31                                                    ,                                                      normal                            ⁢                                                                                                                  ⁢                            cyclic                            ⁢                                                                                                                  ⁢                            prefix                                                                                                                                                                                        l                          ″                                                                                                                                                  CSI                            ⁢                                                                                                                  ⁢                            reference                            ⁢                                                                                                                  ⁢                            signal                            ⁢                                                                                                                  ⁢                            configurations                            ⁢                                                                                                                  ⁢                            0                            ⁢                                                          -                                                        ⁢                            27                                                    ,                                                      extended                            ⁢                                                                                                                  ⁢                            cyclic                            ⁢                                                                                                                  ⁢                            prefix                                                                                                                                ⁢                                                                          ⁢                                                                          ⁢                                      w                                          l                      ″                                                                      =                                  {                                                                                                                                                                        1                                                                                                                      p                                ∈                                                                  {                                                                      15                                    ,                                    17                                    ,                                    19                                    ,                                    21                                                                    }                                                                                                                                                                                                                                                                          (                                                                      -                                    1                                                                    )                                                                                                  l                                  ″                                                                                                                                                                                    p                                ∈                                                                  {                                                                      16                                    ,                                    18                                    ,                                    20                                    ,                                    22                                                                    }                                                                                                                                                                    ⁢                                                                                                  ⁢                                                                                                  ⁢                                                  l                          ″                                                                    =                      0                                        ,                                                                  1                        ⁢                                                                                                  ⁢                                                                                                  ⁢                        m                                            =                      0                                        ,                    1                    ,                    …                    ⁢                                                                                  ,                                                                                            N                          RB                          DL                                                -                                                  1                          ⁢                                                                                                          ⁢                                                                                                          ⁢                                                      m                            ′                                                                                              =                                              m                        +                                                  ⌊                                                                                                                    N                                RB                                                                  max                                  ,                                  DL                                                                                            -                                                              N                                RB                                DL                                                                                      2                                                    ⌋                                                                                                                                                            
TABLE 6.10.5.2-1Mapping from CSI reference signal configuration to (k′, l′) for normal cyclic prefix.Number of CSI reference signals configuredCSI reference signal1 or 248configuration(k′, l′)ns mod 2(k′, l′)ns mod 2(k′, l′)ns mod 2Frame0(9, 5)0(9, 5)0(9, 5)0structure1(11, 2) 1(11, 2) 1(11, 2) 1type 12(9, 2)1(9, 2)1(9, 2)1and 23(7, 2)1(7, 2)1(7, 2)14(9, 5)1(9, 5)1(9, 5)15(8, 5)0(8, 5)06(10, 2) 1(10, 2) 17(8, 2)1(8, 2)18(6, 2)1(6, 2)19(8, 5)1(8, 5)110(3, 5)011(2, 5)012(5, 2)113(4, 2)114(3, 2)115(2, 2)116(1, 2)117(0, 2)118(3, 5)119(2, 5)1Frame20(11, 1) 1(11, 1) 1(11, 1) 1structure21(9, 1)1(9, 1)1(9, 1)1type 222(7, 1)1(7, 1)1(7, 1)1only23(10, 1) 1(10, 1) 124(8, 1)1(8, 1)125(6, 1)1(6, 1)126(5, 1)127(4, 1)128(3, 1)129(2, 1)130(1, 1)131(0, 1)1
For CSI feedback, LTE has adopted an implicit CSI mechanism where a UE does not explicitly report, for example, the complex valued elements of a measured effective channel, but rather the UE recommends a transmission configuration for the measured effective channel. Thus, the recommended transmission configuration implicitly gives information about the underlying channel state.
In LTE, the CSI feedback is given in terms of a transmission rank indicator (RI), a precoder matrix indicator (PMI), and one or two channel quality indicators (CQIs). The CQI/RI/PMI report can be wideband or frequency selective depending on which reporting mode is configured. The RI corresponds to a recommended number of streams that are to be spatially multiplexed and thus transmitted in parallel over the effective channel. The PMI identifies a recommended precoder (in a codebook which contains precoders with the same number of rows as the number of CSI-RS ports) for the transmission, which relates to the spatial characteristics of the effective channel. The CQI represents a recommended transport block size (i.e., code rate) and LTE supports one or two simultaneous (on different layers) transmissions of transport blocks (i.e., separately encoded blocks of information) to a UE in a subframe. There is thus a relation between a CQI and an SINR of the spatial stream(s) over which the transport block or blocks are transmitted.
In LTE Release 10, CSI feedback can correspond to multiple downlink carriers, in which case CSI feedback such as CQI/PMI/RI can be provided for each serving cell corresponding to each of the downlink carriers. In this context, P antenna ports of an antenna configuration of a network node are present on the same serving cell, and a CQI/PMI/RI report for P antenna ports for the cell corresponds to the P antenna ports present on the serving cell.
In LTE Release 11, CSI processes are defined such that each CSI process is associated with a CSI-RS resource and a CSI interference measurement (CSI-IM) resource. A UE in transmission mode 10 can be configured with one or more (up to four) CSI processes per serving cell by higher layers, and each CSI reported by the UE corresponds to a CSI process. A UE may be configured with a RI-reference CSI process for any CSI process, such that the reported RI for the CSI process is the same as for the RI-reference CSI process. This configuration may be used to force a UE to report the same RI for several different interference hypotheses, even though another RI would be the best choice for some hypothesis. Furthermore, a UE is restricted to report PMI and RI within a precoder codebook subset configured for each CSI process by higher layer signaling. This configuration may also be used to force a UE to report a specific rank for a certain CSI process.
Both aperiodic (i.e., triggered by eNB) and periodic CSI reports are supported (known as PA-CSI and P-CSI, respectively). CSI reports are also referred to as CSI feedback, and these terms may be used interchangeably herein. In the CSI process, a set of CSI-RS ports are configured for which the UE performs measurements. These CSI-RS ports are configured to be periodically transmitted with, for example, 5 ms, 10 ms, 20 ms, or any other suitable periodicity. The periodic CSI report uses PUCCH format 2 (or its variants 2a, 2b), has a configured periodicity as well (e.g., 20 ms), and is a narrow bit pipe containing at most 11 bits.
Recent development in 3GPP has led to the discussion of two-dimensional antenna arrays, where each antenna element has an independent phase and amplitude control, thereby enabling beamforming both in the vertical and the horizontal dimensions. Such antenna arrays may be at least partially described by the number of antenna columns corresponding to the horizontal dimension Nh, the number of antenna rows corresponding to the vertical dimension Nv, and the number of dimensions corresponding to different polarizations Np. Thus, the total number of antennas is N=NhNvNp.
FIG. 7 illustrates an example of a two-dimensional antenna array of cross-polarized antenna elements. More particularly, FIG. 7 illustrates an example of an antenna with Nh=4 horizontal antenna elements and Nv=8 vertical antenna elements. It furthermore consists of cross-polarized antenna elements, meaning that the number of polarization states Np=2. Such an antenna can be denoted as an 8×4 antenna array with cross-polarized antenna elements. The right hand side illustrates an example port layout, with 2 vertical ports and 4 horizontal ports, which could for instance be obtained by virtualizing each port by 4 vertical antenna elements. Hence, assuming cross-polarized ports are present, the UE will measure 16 antenna ports in this example.
From a wireless device perspective, however, the actual number of antenna array elements is not visible to the wireless device, but rather the antenna ports, where each port corresponds to a CSI reference signal. The wireless device can thus measure the channel from each of these ports. Therefore, we introduce a 2D port layout, described by the number of antenna ports corresponding to the horizontal dimension Mh, the number of antenna rows corresponding to the vertical dimension Mv, and the number of dimensions corresponding to different polarizations Mp. The total number of antenna ports is thus M=MhMvMp. The mapping of these ports onto the N antenna elements is an eNB implementation issue, and thus not visible by the wireless device. The wireless device does not even know the value of N; it only knows the value of the number of ports M.
Precoding may be interpreted as multiplying the signal with different beamforming weights for each antenna port prior to transmission. A typical approach is to tailor the precoder to the antenna form factor (i.e., taking into account Mh, Mv and Mp when designing the precoder codebook.
A common approach when designing precoder codebooks tailored for 2D antenna arrays is to combine precoders tailored for a horizontal array and a vertical array of antenna ports, respectively, by means of a Kronecker product. This means that (at least part of) the precoder can be described as a function of:WH⊗WV where WH is a horizontal precoder taken from a (sub)-codebook XH containing NH codewords. Similarly, WV is a vertical precoder taken from a (sub)-codebook XV containing NV codewords. The joint codebook, denoted by XH⊗XV, thus contains NH·NV codewords. The codewords of XH are indexed with k=0, . . . , NH−1, the codewords of XV are indexed with l=0, . . . , NV−1, and the codewords of the joint codebook XH⊗XV are indexed with m=NV·k+1 (meaning that m=0, . . . , NH·NV−1).
For Release 12 wireless devices and earlier, only a codebook feedback for a 1D port layout is supported, with 2, 4 or 8 antenna ports. Hence, the codebook is designed assuming these ports are arranged on a straight line.
A method has been proposed to use measurements on fewer CSI-RS ports for periodic CSI reports than measurements for the aperiodic CSI reports. In one scenario, the periodic CSI report framework is identical to the legacy terminal periodic CSI report framework. Hence, periodic CSI reports with 2, 4 or 8 CSI-RS ports are used for the P-CSI reporting, and additional ports are used for the A-CSI reporting. From the UE and eNB perspective, the operations related to periodic CSI reporting is identical to legacy operation. The full, large 2D port layout CSI measurements of up to 64 ports or even more is only present in the aperiodic reports. Since A-CSI is carried over PUSCH, the payload can be much larger than the small 11-bit limit of the P-CSI using PUCCH format 2.
It has been agreed that for 12 or 16 ports, CSI-RS resources for class A (or non-precoded CSI-RS) CSI reporting is composed as an aggregation of K CSI-RS configurations each with N ports. In case of CDM-2, the K CSI-RS resource configurations indicate CSI-RS RE locations according to legacy resource configurations in TS 36.211. For 16 ports: (N,K)=(8,2) or (2,8). For 12 ports: (N,K)=(4,3), (2,6). The ports of the aggregated resource are as follows:                The aggregated port numbers are 15, 16, . . . 30 (for 16 CSI-RS ports)        The aggregated port numbers are 15, 16, . . . 26 (for 12 CSI-RS ports).        
For a given P antenna ports, the Release 10 and 12 precoding codebooks are designed so that the P/2 first antenna ports (e.g., 15-22 for P=16) should map to a set of co-polarized antennas and the P/2 last antenna ports (e.g., 23-30 for P=16) are mapped to another set of co-polarized antennas, with an orthogonal polarization to the first set. For example, the first subset is associated with a first length-P/2 vector of a length-P precoding vector in a codebook. The second subset is associated with a second length-P/2 vector of the length-P precoding vector, wherein the second length-P/2 is obtained by scaling the first length-P/2 vector by a complex number. This is thus targeting cross-polarized antenna arrays, or more generally, antenna arrays with at last two distinct polarization states.
FIG. 8 illustrates the port numbering for P=8 antenna ports. The codebook principles for the rank 1 case are that a DFT “beam” vector is chosen for each set of P/2 ports and a phase shift with QPSK alphabet is used to co-phase the two sets of antenna ports. A rank 1 codebook is thus constructed as:
      (                            a                                                  ae                          i              ⁢                                                          ⁢              ω                                            )     where α is a length P/2 vector that forms a beam for the first and second polarizations, respectively, and ω is a co-phasing scalar that co-phases the two orthogonal polarizations.